Hydraulic pumps of the above type are already known, in which the deaerating means are formed as a valve which enables the trapped air to be drained into a low pressure liquid tank surrounding the capacity. This solution has the drawback that it implies establishement of a low pressure communication between the capacity and the tank, which can in case cause in particular an air suction effect toward the capacity when a level of the liquid in the tank is under a level of the valve.